r The number of processes waiting for run time.
b The number of processes in uninterruptable sleep.
w The number of processes swapped out but otherwise runnable.

Memory

swpd the amount of virtual memory used (kB).
free the amount of idle memory (kB).
buff the amount of memory used as buffers (kB).
cache the amount of memory used as cache (kB).
inact the amount of inactive memory. (-a option)
active the amount of active memory. (-a option)

Swap

si Amount of memory swapped in from disk (kB/s).
so Amount of memory swapped to disk (kB/s).

IO

bi Blocks sent to a block device (blocks/s).
bo Blocks received from a block device (blocks/s).

System

in The number of interrupts per second, including the clock.
cs The number of context switches per second.

These are percentages of total CPU time.

us user time
sy system time
id idle time
wa time spent waiting for IO. included n idle time.
st time stolen from a virtual machine.